Anti corrosion treatment of aluminium or aluminium alloy surfaces

ABSTRACT

An anti corrosion treatment of an aluminium or aluminium alloy surface comprising the steps of treating the surface with a solution having a pH of from greater than 0 to less than 2 and comprising at least one metal oxo ion completed with phosphorus (V) wherein the metal oxo ion is vanadate or tungstate or a mixture thereof.

TECHNICAL FIELD

This invention relates to anti corrosion treatment of aluminium oraluminium alloy surfaces. In particular, the present invention relatesto an anti corrosion treatment of metal coated steel having an aluminiumor aluminium alloy surface.

BACKGROUND ART

Zinc, aluminium and/or combinations of aluminium and zinc (Al/Zn), arewidely used as surface coatings, particularly but not exclusively forsteel for protection against rust and corrosion. In practice, however,the zinc or Al/Zn coatings are susceptible to white rust or black rustrespectively when exposed to the atmosphere due to reactions withmoisture. Such rust is detrimental to the surface and generally makescoated steel substrates unsaleable despite the fact that the overallservice life of the coated steel may remain the same and the formationof rust generally interferes with finishing operations. The ability toresist such corrosion is referred to herein as wet stack performance.

In order to inhibit the formation of rust on coated surfaces it isgenerally accepted that the treatment of a surface with a chromateimparts anti corrosive properties and this type of treatment isgenerally referred to as chromate passivation. However chromate ishighly toxic to exposed workers and due to its high toxicity, disposalof chromium residues is difficult. Further in various markets the yellowdiscolouration of the treated coated surfaces is considered to be anunacceptable product attribute.

In order to overcome the problems associated with chromate passivation,phosphate coatings have been used. However the anticorrosion propertiesof phosphate have been found to be far inferior to the above mentionedchromate treatment.

U.K. Patent Application No. 2,070,073 discloses an anti corrosivetreatment for preventing white rust on galvanized steel comprisingapplying to the surface of a galvanized steel sheet an acidic solutioncontaining molybdic acid or a molybdate in a concentration of 10-200 g/lcalculated as molybdenum and adjusted to a pH of 1 to 6 by addition ofan organic or inorganic acid. However, it has been found that thecorrosion resistance of aluminium and aluminium alloy surfaces treatedwith the above solution is inferior to the chromate treated substratesunder certain conditions and the treated surfaces suffer from anundesirable degree of discolouration. Moreover modybdate treatedsurfaces have been shown to change from a pale yellow/blue to a stronggreen colour when stored for periods of time in excess of 24 hours.

Accordingly it is an object of the present invention to provide meansfor avoiding and/or ameliorating at least some of the above discusseddisadvantages of the prior art.

DISCLOSURE OF THE INVENTION

According to one aspect, the present invention consists in an anticorrosion treatment of an aluminium or aluminium alloy surfacecomprising treating said surface with a solution having a pH of from 0to 2 and comprising at least one metal oxo ion complexed withphosphorous (V) wherein the metal oxo ion is vanadium or tungsten or amixture thereof.

Surprisingly, it has been found that the treated aluminium or aluminiumalloy surfaces of the present invention provide substantial advantagesover the prior art. In particular the vanadate and tungstate treatingsolution of the present invention provides improved high temperaturecorrosion resistance and reduce discolouration of the treated surface.

Ammonium and alkali metal salts such as ammonium vanadate and tungstateor sodium vanadate and tungstate are generally the preferred source ofthe metal oxo ion. Preferably the metal oxo ion is present in aconcentration from 10 to 100 g/l and more preferably from 10 to 60 g/l.

In a highly preferred embodiment of the present invention, typicallyaluminium etching agents are employed during the treating step.Preferred etching agents are those containing at least one fluoridecontaining compound such as sodium fluoride and sodiumtetrafluoroborate. Desirably between 0.4 g/l and 1.5 g/l of fluoride andmore preferably between 0.4 to 0.7 g/l is used.

Phosphoric acid is the preferred source of phosphorus (V) and theconcentration of the phosphoric acid used in the present invention isgenerally from 10% (1.5M) to 50% (7.5M).

Preferably the aluminium or aluminium surface is coated by dipping thesurface to be treated into a containing the solution but the desiredsurface may be treated by other means such as roller coating, spraying,or the like.

Accordingly to a second aspect, the present invention relates to acomposition for anti corrosion treatment of an aluminium or aluminiumalloy surfaces comprising the steps of treating the surface with asolution having a pH of from 0 to 2 and wherein said solution comprises:

(1) at least one metal oxo ion complexed with phosphorus (V) wherein themetal oxo ion is vanadium or tungsten or mixtures thereof and

(2) at least one aluminium or aluminium alloy etching agent.

It has been found that the anti corrosive property of the solutionaccording to the present invention becomes deficient at pH above 2 andas such the pH of the acidified solution according to the presentinvention is preferably lower than 2 and more preferably between 1 and1.5. The pH of the solution according to the present invention isadjusted by the addition of the phosphoric acid.

The bath temperature of the resulting anti corrosion solution of thepresent invention should be such that the reactive ingredients of theacidic solution bond to the metal surface and generally the bath ismaintained between 20° to 80° C. Higher bath temperatures areadvantageous for the chemical reaction and the subsequent drying, butbath temperatures of 30° to 60° C. are preferred as too high temperatureresults in an increase of acid vaporization. The contact time of thesubstrate in the bath is preferably about 1 second or less.

Preferred embodiments of the invention will now be described by way ofexample only with reference to the following examples:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot of the discolouration vs contact time forphosphate/vanadate passivation treatment.

FIG. 2 shows the change in "yellowness" between the phosphate (a),molybdate/phosphate (b), vanadate/phosphate (c) and tungstate/phosphate(d) samples when compared to the unpassivated reference standard.

BEST MODE OF CARRYING OUT THE INVENTION EXAMPLE 1 Effect of Contact Timeand Temperature on Vanadate/Phosphate Passivation Efficiency

A bath containing ammonium vanadate (23.4 g in 5 liters water; 0.04M)sodium fluoride (1.42 g/l 0.34M) was acidified with phosphoric acid (900ml; 2.7M) until the pH of the bath containing the ammonium vanadate andsodium fluoride reached 1.5. A strip of aluminium (53%)/zinc (45%)coated steel was then dipped into the bath for varying contact time atdifferent passivation temperatures. The results of the resulting coatingfilm are given in Tables 1 to 3.

                  TABLE 1                                                         ______________________________________                                        Passivation                                                                   Temperature                                                                              Contact    Corrosion resistance                                    °C. Time       (in wet stack test @ 40° C.)                     ______________________________________                                        40          2         <5% black rust on sample                                                      after 8 weeks testing                                               4         <5% black rust on sample                                                      after 8 weeks testing                                              10         <5% black rust on sample                                                      after 8 weeks testing                                              30         5-10% black rust on sample                                                    after 8 weeks testing                                              45         5-30% black rust on sample                                                    after 8 weeks testing                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Passivation                                                                   Temperature                                                                              Contact    Corrosion resistance                                    °C. Time       (in wet stack test @ 40° C.)                     ______________________________________                                        50          2         <5% black rust on sample                                                      after 8 weeks testing                                               4         <5% black rust on sample                                                      after 8 weeks testing                                              10         <5% black rust on sample                                                      after 8 weeks testing                                              30         <5% black rust on sample                                                      after5 8 weeks testing                                             45         5-50% black rust on sample                                                    after 8 weeks testing                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Passivation                                                                   Temperature                                                                              Contact    Corrosion resistance                                    °C. Time       (in wet stack test @ 40° C.)                     ______________________________________                                        60          2         <5% black rust on sample                                                      after 8 weeks testing                                               4         <5% black rust on sample                                                      after 8 weeks testing                                              10         <5% black rust on sample                                                      after 8 weeks testing                                              30         <5% black rust on sample                                                      after 8 weeks testing                                              45         <5% black rust on sample                                                      after 8 weeks testing                                   ______________________________________                                    

EXAMPLE 2 Effect on Contact Time on Product Discolouration

The acidic treating solution was made according to Example 1 and theeffects on various contact time on product discolouration is given inTable 4 and the results are plotted in FIG. 1.

                  TABLE 4                                                         ______________________________________                                        Passivation                                                                   Temperature   Contact Time                                                                             dL (lightness)                                       ______________________________________                                        40             2         82.28                                                               4         80.95                                                              10         75.79                                                              30         62.58                                                              45         62.24                                                ______________________________________                                    

With reference to FIG. 1, the X-axis represents the various contact timewhile the Y-axis represents the degree of discolouration wherein thegreater value the lighter the appearance of the treated surface. As canbe seen from FIG. 1, longer contact time for a phosphate/vanadatepassivation treatment at 40° C. results in darker discolouration of thetreated surface. As such, reduced contact time is preferred for minimumdiscolouration of the treated surface.

EXAMPLE 3 Effect on Vanadate Concentration on Passivation Efficiency

The acidic coating solution was made according to Example 1 except thevanadate concentration was varied. A metal coated steel strip wascontacted for 4 seconds in the bath and the results are summarized inTable 5.

                  TABLE 5                                                         ______________________________________                                        Passivation                                                                             Vanadate                                                            Temperature                                                                             Concentration                                                                              Corrosion resistance                                   °C.                                                                              (mol/L)      (in wet stack tests @ 40° C.)                   ______________________________________                                        50        0.02         up to 40% black rust within                                                   2 weeks                                                          0.04         <5% black rust on sample                                                      after 8 weeks testing                                            0.04         Insoluble precipitate formed                           ______________________________________                                    

EXAMPLE 4 Effect on Fluoride Concentration on Passivation Efficiency

The acidic coating solution was made according to Example 1 except theconcentration of fluoride was varied. The contact time was 4 seconds andthe passivation temperature was held constant at 30° C. The results aresummarized in Table 6 below.

                  TABLE 6                                                         ______________________________________                                        Fluoride                                                                      Concentration  Corrosion resistance                                           (mol/L)        (in wet stack test @ 40° C.)                            ______________________________________                                        0.017          10-20% blackening                                                             5 weeks testing                                                0.034          <5% black rust on sample                                                      after 8 weeks testing                                          0.068          mild grey discolouration                                                      approximately 10% of sample                                    ______________________________________                                    

EXAMPLE 5 Effect of Contact Time on Tungstate/Phosphate PassivationEfficiency

The acidic coating solution was made according to Example 1 exceptsodium tungstate (0.04M) was used in place of ammonium vanadate and theresults are summarized in Table 7.

                  TABLE 7                                                         ______________________________________                                        Contact     Corrosion resistance (in wet stack tests                          Time (sec)  @ 40 C.                                                           ______________________________________                                         2          approx 10% black rust on sample after                                         8 weeks testing                                                    4          approx 10% black rust on sample after                                         8 weeks testing                                                   10          <10% black rust on sample after                                               8 weeks testing                                                   30          approx 15% black rust on sample after                                         8 weeks testing                                                   45          approx 50% black rust on sample after                                         8 weeks testing                                                   ______________________________________                                    

EXAMPLE 6 Comparison of Product Appearance of Phosphate,Molybdate/Phosphate, Tungstate/Phosphate, Vanadate/Phosphate andChromate Systems

The molybdate/phosphate, vanadate/phosphate and tungstate/phosphatesolutions were prepared according to the general procedure of Example 1.The contact time was 2 to 4 seconds and the passivation temperature washeld constant at 70° C. The results are summarized in Table 8.

                  TABLE 8                                                         ______________________________________                                                       Results for 70° C. wet stack                            Passivation solution                                                                         (4 weeks)                                                      ______________________________________                                        1:1 (7.5M) H.sub.3 PO.sub.4                                                                  5-10% grey staining on the samples                                            2-5 mm edge corrosion                                          0.04M Mo       5-10% grey staining on the samples                             0.03M F.sup.-  3-5 mm edge corrosion                                          7.5M H.sub.3 PO.sub.4                                                         0.04M W        2-10% grey staining on the samples                             0.03M F.sup.-  2-5 mm edge corrosion                                          7.5M H.sub.3 PO.sub.4                                                         0.04M V        2-5% grey staining on the samples                              0.03M F.sup.-  1-3 mm edge corrosion                                          7.5M H.sub.3 PO.sub.4                                                         Chromate controls                                                                            removed after 1 week testing due                                              to excessive black rust on surface                             ______________________________________                                    

The passivation temperature of the samples in Example 6 was heldconstant at 30° C. with a contact time of 2 seconds and the solutionswere prepared such that the pH was less than 1 and the results areillustrated in FIG. 2. With reference to FIG. 2, the ordinate shows thechange in "yellowness" for various treatments indicated as follows onthe co-ordinate axis:

(a)=phosphate

(b)=molybdate/phosphate

(c)=vanadate/phosphate

(d)=tungstate/phosphate

The dB readings are a delta (change) B between the sample and theunpassivated reference standard. Readings were taken with a McBeth 20.20brand integrating sphere colour spectrophotometer and analysed using ahunter lab equation. FIG. 2 shows the lower yellow discolouration of thevanadate/phosphoric acid treatment compared with the others. Thevanadate/phosphoric acid system has a further advantage when comparedwith the molybdate/phosphoric acid system in that the colour of thevanadate/phosphoric acid treated panels does not change with timewhereas the molybdate/phosphoric acid treated panels have been seen tochange from a pale yellow to a strong green colour when stored forperiods up to 10 months.

EXAMPLE 7 Comparison of Corrosion Resistance of Phosphate,Molybdate/Phosphate, Vanadate/Phosphate, Tungstate/Phosphate andChromate Systems

The solutions of Example 6 were used to determine the corrosionresistance of various passivation systems at varying temperatures. Ineach case, the contact time was 2 seconds and the results are givenbelow in Table 9 (samples passivated at 30° C.) and Table 10 (samplespassivated at 50° C.).

                  TABLE 9                                                         ______________________________________                                                         Results after 7 weeks wet stack                              Passivation system                                                                             @ 40° C.                                              ______________________________________                                        Phosphoric acid  50-60% white corrosion product                                                on surface some surface                                                       blackening also evident                                      Molybdate + phosphoric acid                                                                    No blackening evident                                        Vanadate + phosphoric acid                                                                     No blackening evident                                        Tungstate + phosphoric acid                                                                    Light/medium black blotches                                                   seen on up to 10% of sample                                                   surface                                                      Chromate control No corrosion evident                                         ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                                         Results after 7 weeks wet stack                              Passivation system                                                                             @ 40° C.                                              ______________________________________                                        Phosphoric acid  Samples acceptable, but variable                                              amounts of white corrosion                                                    product are evident on some                                                   surfaces                                                     Molybdate + phosphoric acid                                                                    No blackening evident                                        Vanadate + phosphoric acid                                                                     No blackening evident                                        Tungstate + phosphoric acid                                                                    Samples acceptable with a few                                                 small (<2 mm) spots of black                                                  corrosion products evident                                   Chromate control No corrosion evident                                         ______________________________________                                    

Table 11 below contains the results of varying the concentration ofphosphoric acid to 3M while maintaining a contact time of 2 seconds at apassivation temperature of 50° C.

                  TABLE 11                                                        ______________________________________                                                         Results after 2 weeks wet stack                              Passivation system                                                                             @ 40° C.                                              ______________________________________                                        Phosphoric acid  Mild black rust evident over                                                  entire sample                                                Molybdate + phosphoric acid                                                                    Black rust evident on up to 30%                              pH = 1.3         of sample surface                                            Vanadate + phosphoric acid                                                                     No black rust evident                                        pH = 1.35                                                                     Chromate control No black rust evident                                        ______________________________________                                    

Substrates coated with the composition according to the presentinvention shows excellent anti corrosion properties and the long termprevention of rust as well as good adhesion of paint.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

I claim:
 1. An anti corrosion treatment of an aluminium or aluminiumalloy surface comprising the steps of treating said surface with asolution having a pH of from greater than 0 to less than 2 andcomprising at least one metal oxo ion complexed with phosphorus (V)wherein the metal oxo ion is vanadate or tungstate or a mixture thereof.2. An anti corrosion treatment according to claim 1, wherein thesolution further comprises at least one fluoride containing compound. 3.An anti corrosion treatment according to claim 2, wherein the fluoridecontaining compound is selected from the group consisting of sodiumfluoride and sodium tetrafluoroborate.
 4. An anti corrosion treatmentaccording to claim 2, wherein the fluoride containing compound ispresent in a concentration of from 0.4 g/l to 1.5 g/l.
 5. An anticorrosion treatment according to claim 4, wherein the concentration isfrom 0.4 g/l to 0.7 g/l.
 6. An anti corrosion treatment according toclaim 1, wherein the metal oxo ion is vanadate.
 7. An anti corrosiontreatment according to claim 6, wherein the vanadate is present in aconcentration from 10 g/l to 100 g/l.
 8. An anti corrosion treatmentaccording to claim 7, wherein the concentration is from 10 g/l to 60g/l.
 9. An anti corrosion treatment according to claim 1, wherein the pHrange is from 1 to 1.5.